1. Field of the Invention
The invention relates to an in-plane switching mode liquid crystal display device and more particularly, an in-plane switching mode liquid crystal display device having a fast response and an improved transmittance.
2. Related Art
A flat display device such as a liquid crystal display device are frequently used as a display device due to a slim and light structure as well as a lower power consumption. To display a data, a liquid crystal device may utilize an optical anisotropy and a polarity of liquid crystal molecules. The liquid crystal molecules are normally thin and long. The liquid crystal molecules have a pretilt angle such that the molecules are arranged to having a direction. Upon application of a driving voltage to the liquid crystal molecules, the pretilt angle may be changed and the liquid crystal molecules are arranged in a different direction. As a result, images are displayed and light polarized by the optical anisotropy of the liquid crystal is controlled. An active matrix liquid crystal display is often used because of a superior resolution and a picture quality. The active matrix liquid crystal display has a thin film transistor and a pixel electrode connected to the thin film transistor which are arranged in a matrix pattern.
The liquid crystal display device includes a liquid crystal panel having a upper substrate and a lower substrate. The upper substrate is referred to as a color filer substrate, and the lower substrate is referred to as an array substrate. The upper substrate and the lower substrate are spaced apart from each other and a liquid crystal is interposed therebetween. The upper substrate includes a common electrode and the lower substrate includes a pixel electrode. When a driving voltage is supplied to the common electrode on the upper substrate and the pixel electrode on the lower substrate, an electric field, which is perpendicular to the electrodes, is formed by a difference of the supplied voltage. The electric field controls the arrangement direction of the liquid crystal molecules. When the liquid crystal is driven by the perpendicular electric field, a transmittance and an aperture ratio may increase but a viewing angle may decrease. Accordingly, a driving method of a liquid crystal using in-plane switching (IPS) with a horizontal electric field is used.
FIG. 1 is a cross-sectional view illustrating a liquid crystal panel 1 of an in-plane switching mode liquid crystal display device according to the related art. The liquid crystal panel 1 includes a color filter substrate 9 having a color filter and an array substrate 10 having a thin film transistor. The color filter substrate 9 and the array substrate 10 are configured to face each other. A liquid crystal layer 11 is disposed between the color filter substrate 9 and the array substrate 10. A common electrode 17 and a pixel electrode 30 are disposed horizontally on the array substrate 10, and a horizontal electric field L is formed by a difference of voltage supplied to the common electrode 17 and the pixel electrode 30. Accordingly, the in-plane switching mode liquid crystal display device operates to control the liquid crystal molecules within the horizontal electric field L.
FIGS. 2A and 2B illustrate ‘on’ and ‘off’ states of the in-plane switching mode liquid crystal display device according to the related art. FIG. 2A illustrates an arrangement state of a liquid crystal in an ‘on’ state. A voltage is supplied and the arrangement of each liquid crystal 11a corresponding to a common electrode 17 and a pixel electrode 30 remains unchanged. Liquid crystals 11b between the common electrode 17 and the pixel electrode 30 are arranged in the same direction of a horizontal electric field L. The horizontal electric field L is formed by a voltage supplied between the common electrode 17 and the pixel electrode 30. Accordingly, the in-plane switching mode liquid crystal display device has a broad viewing angle because the arrangement of the liquid crystal is changed in response to the horizontal electric field. As a result, the in-plane switching mode liquid crystal display device may be viewed from top, bottom, left and right positions with an angle of about 80° to 85° without a reversal process. FIG. 2B illustrates an arrangement state of a liquid crystal in an ‘off’ state. No voltage is applied and the arrangement state of a liquid crystal layer 11 is unchanged. No horizontal electric field between the common electrode 17 and the pixel electrode 30 is formed.
FIG. 3 is a plan view of an in-plane switching mode liquid crystal display device according to the related art. The common electrode 17 and the pixel electrode 30 are formed on the array substrate 10. The common electrode 17 and the pixel electrode 30 may shield a pixel region P, so that an aperture ratio decreases. As the amount of light passing through the liquid crystal display device is reduced with the electrodes, a brightness may decrease.
A twisted nematic (“TN”) liquid crystal may be often used with a liquid crystal display device and an in-plane switching mode liquid crystal display device. Because the TN liquid crystal has a response time of over 30 ms, which is relatively slow, the liquid crystal display device using the TN liquid crystal may present a low display quality that develops an afterimage in implementing a fast movement display such as an animation. To improve the response speed, a ferroelectric liquid crystal (“FLC”) mode liquid crystal display device is used. The FLC mode liquid crystal display device uses a ferroelectric liquid crystal having a superior response speed.
The ferroelectric liquid crystal is referred to as a chiral smectic C liquid crystal. The response time of the ferroelectric liquid crystal molecules is fast. Each layer of the chiral smectic C liquid crystal is arranged with an angle. When an electric field applies to the chiral smectic C liquid crystal, a dipole moment is arranged in one direction, and a molecular alignment is uniform and is maintained after the electric field is eliminated. Further, when an electric field is supplied in an opposite direction to the chiral smectic C liquid crystal, the molecular alignment may be reversed in an opposite direction at a high speed. The molecular alignment of the ferroelectric liquid crystal differs according to a polarization of an electric field, and the FLC mode liquid crystal display device shows the fast response.
FIG. 4 is a cross-sectional view illustrating a FLC mode liquid crystal display device using a ferroelectric liquid crystal according to the related art. As shown in FIG. 4, a ferroelectric liquid crystal 80 with a gap d1 is disposed between a first alignment layer 55 on an array substrate 50 and a second alignment layer 75 on a color filter substrate 70. In the FLC mode liquid crystal display device, the gap d1 of a liquid crystal panel 40 should be smaller than 2 μm. Further, it may be difficult to inject the ferroelectric liquid crystal 80 because it has almost gel state at the normal temperature.
Accordingly, there is a need of a liquid crystal display that provides advantages of an FLC mode liquid crystal display device and an in-plane switching mode liquid crystal display device such as a broad viewing angle, high brightness, and a fast response velocity.